Given the increasingly severe environmental and ecological constraints, the recovery of waste or residues, irrespective of their origin, has become all the more pertinent. Waste is generally incinerated to form two types of residues: solid non-combustible materials, called clinker or slag, containing high concentrations of pollutants such as heavy metals, which may be released when exposed to water; and residues from the smoke treatment, which are made up of ash from dust removal and smoke detoxification residues from the gas treatment. The recovery of these types of waste may be complex, depending on their composition. Some of these are used in construction or in the preparation of bituminous mixtures.
For example, U.S. Pat. No. 5,521,132 describes the recovery of ash via the production of ceramic materials. The ash is brought into contact with borax and a calcium compound, and is then heated to very high temperatures (approximately 1000° C.) to form ceramics.
EP 0 743 079 also describes the treatment of ash from waste incineration, which is used to stabilise the heavy metals in the ash. The ash is subjected to a phosphating reaction at very high temperatures (between 500° C. and 1200° C.) in order to transform the toxic metal chlorides into phosphate salts.
Moreover, WO 97/31874 mentions a method of making ash inert via its reaction with a phosphate mixture in order to form a paste that is fully carbonised at a temperature of more than 600° C. The resulting residue is mixed with water and a hydraulic binder such as cement.
These different processes mainly aimed at encapsulating or confining the heavy metals present in the ash, ceramics or cement, in order to prevent them from spreading in the environment. These processes therefore resulted in a significant increase in the mass of the waste, without adding to the value of their constituent elements, which are simply trapped in the cement or the ceramics before being disposed of in specific landfills.
In addition, JP H11-33594 shows us a process for treating sewage sludge via a phosphoric acid solution at a temperature of 40° C. This process is not optimal for purifying sewage sludge.
This invention seeks to overcome these drawbacks and recover at least a part of the constituents of the ash obtained from waste incineration.